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Chapter 12
Note: If your segmented, time-dependent predictor is constant within segments, as in the blood
pressure example given above, it may be easier for you to specify the piecewise-constant,
time-dependent predictor by splitting subjects across multiple cases. See the discussion on
Subject Identifiers in Complex Samples Cox Regression on p. 74 for more information.
In the Define Time-Dependent Predictor dialog box, you can use t he function-building controls
to build the expression for the time-dependent covariate, or you can enter it directly in the
Numeric Expression text area. Note that string constants must be enclosed in quotation marks
or apostrophes, and numeric constants must be typed in American format, with the dot as the
decimal delimiter. The resulting variable is given the name you specify and should be included
as a factor or covariate on the Predictors tab.
Subgroups
Figure 12-5
Cox Regression dialog box, Subgroups tab
Baseline Strata. A separate baseline hazard and survival function is computed for each value of
this variable, while a single set of model coefficients is estimated across strata.
Subpopulation Variable. Specify a variable to define a subpopulation. The analysis is performed
only for the selected category of the subpopulation variable.